1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive for writing micro-jog values along the centerline of a reserved track and for writing user data offset from the centerline of a data track.
2. Description of the Prior Art
FIG. 1 shows a disk 2 having an exemplary prior art disk format comprising a plurality of concentric tracks 4, where each track 4 comprises a plurality of embedded servo sectors 6. A head is actuated radially over the disk 2 in order to write and read user data along circumferential paths defined by the tracks 4. Each embedded servo sector comprises a preamble field 8 for use in synchronizing timing recovery and gain control circuitry, and a sync mark 10 for use in discerning symbol boundaries of servo data 12. At the end of each servo sector 6 are a plurality of servo bursts 14 (A, B, C, D) which are aligned at predetermined offsets from one another and which define a centerline of the track 4. As the head reads the servo bursts 14, an indication of the head""s position relative to the track""s centerline is derived and used by a servo control system to maintain the head along the desired circumferential path during write and read operations.
Magneto-resistive (MR) heads comprise a MR read element spaced apart from an inductive write element. Due to the skew angle of the MR head relative to the concentric tracks recorded on the disk, the read element may not align circumferentially with the write element. At very high recording densities (tracks-per-inch (TPI)), the radial offset between the read and write elements may range from a fraction of a track to several tracks depending on the geometries and radial position of the head. When writing data to the disk, the read element is maintained over a centerline of a xe2x80x9creadxe2x80x9d track while the write element writes data to a xe2x80x9cwritexe2x80x9d track (the read and write tracks may be the same track). Due to the radial offset between the read and write elements, the data may be written along a circumferential path that is offset from the centerline of the write track. During a read operation, the read element is maintained over the circumferential path of the recorded data by introducing an offset value referred to as a xe2x80x9cmicro-jogxe2x80x9d into the servo control loop. The micro-jog corresponds to the offset of the recorded data from the track""s centerline.
FIG. 2A shows a prior art disk drive comprising a disk 18 having a plurality of data tracks and at least one reserved track 40. The disk drive 16 further comprises a position error generator 20 for generating a position error signal (PES) 22 in response to the servo bursts 14 recorded in the servo sector 6, and a micro-jog value 24 stored in a semiconductor memory 26. A servo compensator 28 processes the PES 22 to generate a control signal 30 applied to an actuator (VCM) 32 in order to actuate a head 34 radially over the disk 18. The head 34 comprises a read element 36 offset from a write element 38 (e.g., a magnetoresistive head). When writing data to a selected track 40 (data track or reserved track), the head 34 is positioned such that the read element 36 is aligned over the centerline of a read track 42 (using the servo bursts 14) and the write element 38 is aligned over the selected track 40 offset from the selected track""s centerline. When reading the recorded data from the selected track 40, the head 34 is positioned such that the read element 36 is aligned over the recorded data (offset from the selected track""s centerline) by introducing the micro-jog value 24 into the servo loop. The micro-jog value 24 is selected from the semiconductor memory 26 based on the track number 44 of the selected track.
During manufacture of the disk drive 16, the micro-jog values 24 for each track are typically determined by executing a calibration procedure. A test pattern is written to a selected track and, upon read back, the read signal is evaluated to determine the centerline offset of the recorded data. The micro-jog values 24 (or parameters for calculating the micro-jog values) are recorded in a reserved track 40 (or tracks) on the disk 18. When the disk drive 16 is powered on, the micro-jog values 24 are read from the reserved track(s) 40 and stored in the semi-conductor memory 26.
In order to successfully retrieve the micro-jog values from the reserved track(s) during the power-on sequence, an initial micro-jog value is needed in order to align the read element over the data recorded in the reserved track(s). The initial micro-jog value may be determined during the power-on sequence by reading the reserved track(s) using a nominal micro-jog value. If the read is unsuccessful, the micro-jog value is incrementally modified and another attempt is made to read the reserved track(s). The disk drive iterates this process until the micro-jog value which enables reading the reserved track(s) is discovered. Attempting to read the reserved track(s) by iteratively converging on the correct micro-jog value can significantly increase the duration of the power-on sequence, a problem that is exacerbated by a reserved track(s) recorded with marginal fidelity due, for example, to defects in the recording medium. To recover data recorded in a marginally if recorded track, the disk drive typically executes a retry procedure wherein various parameters (e.g., read channel parameters) are xe2x80x9cfine tunedxe2x80x9d and the data reread until successfully recovered. When attempting to read a marginally recorded reserved track, the disk drive may need to execute the iterative retry procedure for each iteration of the incrementally modified micro-jog values, potentially requiring numerous revolutions to successfully recover the data recorded in the reserved track. This problem can be avoided by storing the initial micro-jog value(s) in a non-volatile semiconductor memory during manufacture of the disk drive, however, this leads to an undesirable increase in the cost of the disk drive.
Thus, in a disk drive employing a head having a read element offset from a write element, there is a need to minimize the duration of a power-on sequence by minimizing the number of revolutions required to read micro-jog values recorded in reserved track(s).
The present invention may be regarded as a disk drive comprising a disk having a plurality of data tracks and at least one reserved track. Each of the data tracks and the at least one reserved track comprise a plurality of embedded servo sectors. Each embedded servo sector comprises a plurality of servo bursts which define a centerline of each track. At least one of the data tracks is for recording user data along a circumferential path offset from the centerline of the data track. The at least one reserved track for recording data track micro-jog values along a circumferential path substantially aligned with the centerline of the reserved track. The disk drive further comprising a head having a read element and a write element, wherein the read element is offset from the write element, and an actuator, responsive to a control signal, for actuating the head radially over the disk. A position error generator within the disk drive, responsive to the servo bursts recorded in the embedded servo sectors and the data track micro-jog values recorded in the at least one reserved track, generates a position error signal (PES) representing a position error of the read element with respect to a circumferential path of data recorded in a selected one of the data tracks. A servo compensator, responsive to the PES, generates the control signal applied to the actuator.
In one embodiment, during a power-on sequence the servo compensator generates the control signal to align the read element over the centerline of the reserved track in order to read the data track micro-jog values recorded in the reserved track. When writing user data to a selected one of the data tracks, the servo compensator generates the control signal to align the read element along a centerline of a read track while the write element writes data to the selected data track along a circumferential path that is offset from the centerline of the selected data track. When reading data from the selected data track, the servo compensator uses at least one of the data track micro-jog values corresponding to the selected data track to generate the control signal to align the read element along the circumferential path of the data recorded in the selected data track.
The present invention may also be regarded as a method executed by a disk drive comprising a disk having a plurality of data tracks and at least one reserved track, and a head comprising a read element offset from a write element, wherein the method for recording data track micro-jog values in the reserved track. The method comprises the steps of calibrating a reserved track micro-jog value, seeking the head to the reserved track, using the reserved track micro-jog value to maintain the read element offset from a centerline of a read track in order to maintain the write element over a centerline of the reserved track, and writing the data track micro-jog values to the reserved track substantially along the centerline of the reserved track.
The present invention may also be regarded as a method executed by a disk drive comprising a disk having a plurality of data tracks and at least one reserved track, and a head comprising a read element offset from a write element, wherein the method for reading data track micro-jog values recorded in the reserved track. The method comprises the steps of seeking the head to the reserved track, maintaining the read element over a centerline of the reserved track, reading the data track micro-jog values recorded in the reserved track, and storing the data track micro-jog values in a semiconductor memory.